I can sign in with my other 4 acounts on thunderbird

Are you trying to add the gmail account ? If yes it's gmail then check all the following is set up: I'm going to assume you set up an '''IMAP''' account, so please advise if POP. Check some settings on computer first: '''Firewall''' It's controlled by either the default that came with computer or by an Anti-Virus you installed. Access the Firewall and make sure the Thunderbird program is set up an allowed program. Don't disable Firewall as that does not help and another may auto function instead. You need the firewall to allow Thunderbird program. * Confirm the Firewall has the Thunderbird program set as allowed. '''VPN''' Have you enabled a VPN? Gmail does not like VPN. It cannot see your IP and thinks you might be a hacker. Some AV products have a habit of auto enabling them. Please double check as some people were surprised to discover they had VPN. Switch it off. * Confirm no VPN is running - and tell me if it was enabled. '''Localhost:''' Check to see if you have something using the localhost. As an example, many people who manage websites may use software like Apache Xampp. Those programs need to be switched off until the gmail account has been set up with oauth token. So it can be switched on again afterwards. * Confirm - anything using localhost is switched off. Thunderbird needs cookies enabled so check the following; In Thunderbird * Settings > Privacy and Security Web Content * Select 'Accept cookies from sites' * Accept third party cookies: always * click on Exceptions button * Add the following : https://accounts.google.com * set status as 'Allow' * click on 'Save Changes' Whilst in that area - scroll down to Passwords section * Click on 'Saved Passwords' * Click on 'Show Passwords' If you see any lines for the gmail account: They may be 'imap://accounts or smtp://account or oauth:// * whatever lines you see for the problem account - remove them as you need to reset this. * Click on 'Close' Exit Thunderbird. Wait a few moments for all background processes to complete. Start Thunderbird At this point I'm assuming you have not created the gmail account. It would be helpful if you created a batch of screenshots for each window becaue if things do not work then you have a documented process in image form which you can then upload. Menu app > New Account > Email Enter name and email address click on Continue and follow instructions These are the settings you would require Incoming * Server name: imap.gmail.com * Port: 993 * User name : full gmail email address * Connection Security: SSL/TLS * Authentication Method: OAuth2 SMTP outgoing Server settings: * Description : full gmail email address * Server name: smtp.gmail.com * Port: 465 * Connection Security: SSL/TLS * Authentication Method: OAuth2 * User name : full gmail email address You should get a prompt from gmail asking for password (same one you use to access the gmail webmail account) - then it will ask you to allow Thunderbird - yes do it. Token should get saved in the Saved Passwords section. If you get any error message -please post images to show the process you used and the results.